Organic compounds have various material series compared with inorganic compounds, and so there is a possibility that various functional materials can be synthesized by an appropriate molecular design. In addition, a formation of organic compounds, for example, a film has excellent flexibility, and becomes adequately workable by polymerization. Therefore, photonics and electronics, each of which is formed by functional organic materials, have attracted attention in recent years.
For example, as an example of photoelectronics using organic semiconductor materials as functional organic materials: a solar battery or an electroluminescent device (also referred to as an organic electroluminescent device) is nominated. These devices utilize electrical properties (carrier transportation properties) and photophysical properties (light absorption or light emission properties) of organic semiconductor materials. Above all, an electroluminescent device has achieved remarkable development.
The basic structure of an electroluminescent device was provided by C. W. Tang et al. in 1987. The device, which is one type of a diode, has the structure composed of an organic thin film with an overall thickness of approximately 100 nm formed by stacking layers of organic compounds having hole transportation properties and organic compounds having electron transportation properties, and a pair of electrodes interposing the organic thin film. Light emission can be observed from the compounds having electron transportation properties formed by light-emitting materials (phosphorescent materials) by applying voltage to the device. (For example, refer to C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol.51, No.12, 913-915 (1987) [Reference 1].)
Hereby, the combination of hole transporting materials and an electron transporting materials is essential for the device containing organic semiconductor materials, especially, a solar battery or an electroluminescent device which employs heterojunction.
However, hole transporting materials account potentially for a large share of organic semiconductor materials in general. With respect to the absolute value of the carrier mobility, the hole mobility of hole transporting materials is several orders of magnitude larger than the electron mobility of electron transporting materials. Therefore, electron transporting materials having excellent electron transportation properties have been hoped.
Further, as electron transporting materials, it has been reported that quinoxaline derivatives which are known as having electron transportation properties are binary-quantificated to improve the thermal physical properties. (For example, refer to Unexamined Patent Publication No. 6-207169 [Reference 2].)
However, electron transportation properties are deteriorated by the binary quantification since interaction of molecules is weakened. In addition, physical properties such as the energy gaps of the quinoxaline derivatives are diverged largely from those of original quinoxaline derivatives.
It has also been disclosed that thermal physical properties (glass transition point or melting point) are improved by introducing a condensed ring into a quinoxaline skeleton to form an adamant plane structure. (For example, refer to Unexamined Patent Publication No. 9-13025 [Reference 3].)
However, although the materials have high thermal physical properties, the materials have demerit of being difficult in maintaining amorphous state and being susceptible to be crystallized.
In addition, as electron transporting materials, materials having hole blocking properties, (which is referred to as hole blocking material especially in this case) are known. In this instance, wide ranging applications become possible since the hole blocking material has a function of blocking holes in addition to a function of transporting electrons. For example, it has been reported that, by interposing a hole blocking material between a hole transporting layer and an electron transporting layer, holes are trapped into the hole transporting layer, and carriers in the hole transporting layer are selectively recombined, then light is generated in an electroluminescent device. (For example, refer to Yasunori KUIMA, Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, vol. 38, 5274-5277 (1999) [Reference 4])
In addition, it has been reported that high efficient light emission can be obtained by using hole blocking materials for forming a triplet light-emitting device. (For example, refer to D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, vol. 74, No. 3, 442-444 (1999) [Reference 5].)
Though a triplet light-emitting device is an effective art for a high efficient electroluminescent device, the electroluminescent device cannot generate light efficiently without using a hole blocking material. Consequently, the hole blocking material becomes an important key.
Hence, hole blocking materials have great importance among an electron transporting material; however, the kinds of materials having both excellent electron transportation properties and excellent hole blocking properties are strictly limited in the present situation. As one of a few examples, BCP (bathocuproin) can be used, which is used in References 4 and 5. However, the BCP deposited film is susceptible to be crystallized, and has significantly adverse effects on the reliability of devices in case of utilizing the BCP for devices actually.
Therefore, among an electron transporting material, a hole blocking material which has excellent hole blocking properties, has excellent film quality, and is hardly crystallized has been hoped.